The invention relates to a process for reducing a content of C1-C2-aldehydes formed, by the action of free radicals, from compounds containing xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups.
It is known that, by virtue of their preparation, aqueous polymer dispersions or solutions may contain aldehyde derivative groups of monomers, crosslinkers or initiators or components thereof, e.g. N-methylol groups, esterified or etherified N-methylol groups, which can release certain amounts of C1-C2-aldehydes such as formaldehyde during storage or use or in the presence of acid. The corresponding dispersions or solutions, because of their content of free formaldehyde, are not used for a whole series of products, such as for sanitary products. It is known to add to such aldehyde-liberating products aldehyde scavengers which are able to bind the released aldehyde and in particular the formaldehyde and thus make the products more acceptable from a toxicological viewpoint (see e.g. EP-A 80635, EP-B 143175, EP-B 492378, EP-A 527411). It is also known (EP-A 492 378, EP-A 505959), to remove undesired amounts of formaldehyde or acetaldehyde by oxidation reactions.
It has now been found that it is not only aqueous polymer dispersions or solutions which contain aldehyde derivative groups of said type which are able to release C1-C2-aldehydes such as formaldehyde, but that, for example, aqueous polymer dispersions and solutions which, as emulsifiers or protective colloids, comprise compounds containing xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups in the molecule (where R=H or C1-C4-alkyl radical) can also do this when they have been exposed to the action of a free radical, for example during their preparation. Compounds containing said groups are, for example, nonionogenic ethoxylated C6-C20-alcohols, C6-C20-amines or C4-C18-alkylphenols having a degree of ethoxylation of from 1 to 100 mols of ethylene oxide, polyethylene oxide having from 30 to 8000 mols of ethylene oxide in the molecule, block copolymers of polyethylene oxide and polypropylene oxide or polyvinyl alcohols, such as polyvinyl esters, the ester groups (e.g. acetate groups) of which have been from 50 to 98 mol % hydrolyzed. The ethoxylated alcohols, amines or alkylphenols can here also be in ionogenic, e.g. sulfated or phosphonated, form or in a salt form thereof. Other ethoxylated compounds containing polyether chains which are used in the preparation of polymer dispersions are, for example, ethoxylated hydroxyethylcelluloses, ethoxylated starch derivatives and ethoxylated monomers, such as ethoxylated 2-hydroxyethyl methacrylate. An overview of compounds having such structures which can be used in the polymerization is given, for example, in N. Schxc3x6nfeld, Grenzflxc3xa4chenaktive xc3x84thylenoxid-Addukte [Surface-active ethylene oxide adducts], Stuttgart 1976 and the supplementary volume from 1984, and Houben-Weyl, Vol. XIV/1, Stuttgart 1961, pages 190-208. These compounds containing xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 and/or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups, which have for decades been largely used industrially in the production of aqueous dispersions and solutions, have for a long time been regarded as wholly acceptable from a health point of view since the release of aldehyde therefrom has been masked by other aldehyde-liberating reactions. Although the literature states that high-energy rays, oxygen or peroxides are able to cause polyethylene oxide or polypropylene oxide chains to undergo a large number of reactions which can lead to degradation or, as a consequence of crosslinking reactions, also to an increase in the molecular weight (see, for example, Kaczmarek et al., Macromol. Symp. 84 (1994) 351-363; Janik, Abh. Akad. Wissenschaften DDR, Akademieverlag Berlin 1987, 551-555; Hidata et al. Yukagaku 1990, 39 (11), 963-966, C.A. 114: 8577m), the fact that such products, by the action of free radicals, can release traces of formaldehyde, acetaldehyde and/or glyoxal, with its consequences was not recognized.
In order to explain the degradation of such groups, it should be pointed out that in an ether chain H atoms in a CHxxe2x80x94Oxe2x80x94 group can be readily abstracted by free radicals. Following H transfer, the free radical center is on the ethylene oxide chain, which can now undergo consecutive reactions, such as a monomer addition (emulsifier grafting), a combination with other free radicals or a degradation of the polyether chain. The latter leads to the formation of low molecular weight compounds. Whenever a xe2x80x94CHxxe2x80x94Oxe2x80x94 compound encounters a free radical or a free-radical formation mechanism, a free-radical-induced degradation can take place, even in the case of ethylene glycol, ethylene glycol dimethyl ether or polyvinyl alcohol. The fact that during such a free-radical-induced degradation significant amounts of formaldehyde, acetaldehyde and/or glyoxal can be formed is a new finding.
Since formaldehyde, acetaldehyde and/or glyoxal are undesired even in low concentrations in products, can lead to discolorations of the products and, in particular, the presence of formaldehyde in products leads to toxicological reservations on the part of the processors, the novel aim was to reduce, in materials, dispersions or solutions which contain xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 (R=H, C1-C4-alkyl) or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups or compounds containing such groups, the content of small amounts of C1-C2-aldehydes, produced by the action of free radials, if at all possible to amounts below 1 ppm. This object is of particular importance in the field of the use of plastics dispersions, since, for example for the preparation of nonwovens for the sanitary and hygiene sector, regulations only permit the use of formaldehyde-free plastics dispersions in certain areas (cf. EP-A 143175, page 2, line 55 ff.).
We have found that this object can be achieved by a process for reducing a content of C1-C2-aldehydes formed, by the action of free radicals, from organic compounds containing xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups (R=H, C1-C4-alkyl), in materials, dispersions or solutions which contain such groups or compounds with such groups, by adding compounds which react with the aldehyde. Preferred compounds which are added as compounds which react with aldehydes are nitrogen-containing compounds which bind aldehyde groups, such as urea or cyclic urea derivatives, or compounds which oxidize or reduce aldehyde groups. It was surprising that the content of C1-C2-aldehydes can be reduced by this method to amounts of 1 ppm and below, i.e. or GB 2086929. Water soluble copolymers containing amide groups are used in EP-A 527411 for reducing the formaldehyde content, and copolymers which contain copolymerized monomers with cyclic urea groups are also suitable for this purpose (EP-A 488605). Measures for reducing the formaldehyde content were and are also used widely in the field of particleboard processing (cf. GB 2086929, EP-A 341569) and, in particular, for removing formaldehyde from textiles treated (impregnated) with aqueous polymer dispersions or aqueous solutions of aminoplastic condensates (U.S. Pat. No. 3,590,100, U.S. Pat. No. 3,957,431). As well as urea and carbamates, cyclic compounds containing NH groups, such as compounds of the pyrrolidone type, and also benzimidazole compounds (U.S. Pat. No. 4,127,382) are also used as formaldehyde scavengers. The reduction in the amount of formaldehyde in aqueous dispersions can, according to EP-A 492378, also be effected by the action of peroxy compounds, such as hydrogen peroxide, organic peroxides, perborates, percarbonates, persulfates or perphosphates, although for the present intended process the use of aldehyde scavengers, i.e. of substances which bind the aldehyde, is preferred.
For a new objective and for other starting materials the measure to be carried out, of binding or chemically modifying C1-C2-aldehydes by means of additives, is in principle known per se, and can thus be carried out in accordance with the details in the prior art. The materials, dispersions or solutions to be treated according to the process always are or contain organic compounds containing xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 (where R=H or C1-C4-alkyl) and/or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups, from which, by the action of free radicals, such as in the case of a degradation of free radical initiators, small but unwanted amounts of C1-C2-aldehydes can form. Materials which contain such groups are, for example, polyvinyl alcohols, alkylene glycols having from 2 to 4 carbon atoms, polymers thereof and derivatives of such compounds, such as ethylene glycol dimethyl ether, or alkylene oxide and, in particular, ethylene oxide addition products with, for example, long-chain alcohols, phenols, amines etc., as are widely used as emulsifiers and protective colloids. Reference is made to examples of such compounds given above. The novel process is of particular importance in the treatment of dispersions and solutions comprising compounds containing said groups which act as emulsifiers and/or protective colloids in the preparation and/or use of polymer dispersions. For example, it is important for the treatment in accordance with the process of aqueous polymer dispersions or solutions which are prepared by free-radical polymerization and/or free-radical after polymerization of olefinically unsaturated monomers in the presence of organic compounds containing xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 and/or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups. Since the novel process is intended to reduce small amounts of C1-C2-aldehydes formed, by the action of free radicals, from xe2x80x94CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups to contents of below 1 ppm if at all possible, it is naturally unwise for the materials, dispersions or solutions to be treated to contain further compounds which are known to be able to release C1-C2-aldehydes. For example, the materials, dispersions or solutions to be treated according to the process should not contain monomers, monomer units, initiators and/or crosslinkers containing aldehyde derivative groups, such as N-methylol groups, nor etherified or esterified N-methylol groups, which, as is known, readily release aldehyde groups.
The examples below show that emulsifiers, protective colloids and block copolymers which contain ethylene oxide groups in bonded form are degraded in the presence of free radicals, forming C1-C2-aldehydes, in particular formaldehyde or acetaldehyde. The same is true of a free-radical action on monomer emulsions which comprise compounds containing ""CH2xe2x80x94CHRxe2x80x94Oxe2x80x94 or xe2x80x94CH2xe2x80x94CH(OH)xe2x80x94 groups (R=H, C1-C4-alkyl) in the emulsifier, protective colloid or monomer.
Formaldehyde is determined as follows:
Method 1: Determination of formaldehyde in the presence of other carbonyl compounds was carried out using a combination of HPLC separation of formaldehyde and chromatographic post column derivatization using acetylacetone, the lutidine derivative forming being quantified against an external calibration. The method is described inter alia by H. Engelhardt and R. Klinckner in the article xe2x80x9cDetermination of Free Formaldehyde in the Presence of Donators in Cosmetics by HPLC and Post Column Derivationxe2x80x9d in Chromatographia Vol.20, No.9 (1985) 559-565 and by U. Schxc3xa4fer-Lxc3xcdderssen and M. Mauxcex2 in the article xe2x80x9cDetermination of Free Formaldehyde in Nonwovens in the Presence of Glyoxalic Acid and Aldehydesxe2x80x9d in Chromatographia Vol.29, No.1/2 (1990) 21-23.
Method 2: A sample of the substance to be investigated is reacted with 2,4-dinitrophenylhydrazine, and the obtained hydrazones are separated by HPLC on an RP phase (Licrospher 100 RP 18.5 xcexcm, (125xc3x974 mm), H2O/acetonitrile eluent, flow rate 1.0 ml/min. detection at 370 nm). The product peak was identified using reference substances.